Stress Relief for Crosslinked Polymers

ABSTRACT

The invention provides methods for inducing reversible chain cleavage of polymer chains in a crosslinked polymeric material or during polymerization of a polymeric material. Reversible cleavage of the polymer backbone is capable of relieving stress in the polymeric material as the bonds reform in a less stressed state. The invention also provides mixtures for making crosslinked polymeric materials, methods for making polymeric materials capable of reversible chain cleavage, materials made by the methods of the invention, and linear monomers containing reversible chain cleavage groups which are useful in the materials and methods of the invention. The mixtures of the invention may be dental restorative compositions used for forming dental restorative materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part application of U.S.application Ser. No. 13/109,400, filed May 17, 2011, which is acontinuation application of U.S. patent application Ser. No. 11/815,914,now U.S. Pat. No. 7,943,680, which is the U.S. National Stage ofInternational Application PCT/US06/04756, filed Feb. 10, 2006, which inturn claims the benefit of U.S. Provisional Application No. 60/651,737,filed Feb. 10, 2005 and U.S. Provisional Application No. 60/763,564,filed Jan. 31, 2006, all of which are incorporated by reference in theirentirety to the extent not inconsistent with the disclosure herein.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grant numberDE010959 awarded by the National Institutes of Health. The governmenthas certain rights in the invention.

BACKGROUND

This invention is in the field of crosslinked polymers, in particularmethods for stress-relief in crosslinked polymers and materialsresulting from the methods.

Cross-linked, gelled polymers have an “infinite” molecular weight andare described as thermosets, implying a network that cannot be melted ormolded (Billlmeyer, F. W., 1984, Textbook of Polymer Science, 3^(rd)ed., Wiley, New York 436). This description is true for most chemicallycross-linked polymers; however several cross-linked networks are knownto undergo bond cleavage or depolymerization at high temperatures orunder various chemical or other treatments (Adhikari, B. et al., Prog.Polym. Sci., 2000, 25, 909). Although such treatments are useful forrecycling purposes, there is an associated degradation in the mechanicalproperties of the polymers.

“Crack-healing” networks, such as those that use groups in the polymerbackbone able to undergo thermoreversible Diels-Alder reactions, areable to relieve stress without mechanical degradation (Chen, X. et al.,2002, Science, 295, 1698). These reactions must be performed at elevatedtemperatures, making them unsuitable for thermally sensitiveapplications.

Reversible bond cleavage is also known to occur during somepolymerization processes. Some controlled free radical polymerizationtechniques employ degenerative chain transfer agents such as reversibleaddition-fragmentation chain transfer (RAFT) agents and iniferters. Aspart of the RAFT process, the RAFT agent adds to an active chain. Atthat point, the RAFT agent may fragment, with part of the RAFT agentremaining attached to the end of the chain (now dormant) and the otherpart forming a new radical. Similarly, in processes employinginiferters, iniferter fragments attached to chains are attached at thechain ends.

BRIEF SUMMARY

In one aspect, the invention provides methods for inducing reversiblecleavage of polymer chains in a polymeric material or duringpolymerization to form a polymeric material. Reversible cleavage of thepolymer backbone is capable of relieving stress as the bonds reform in aless stressed state. The methods of the invention can be applied to avariety of polymeric materials, including, but not limited to, polymericcoatings, dental materials, fiber reinforced materials, and opticalmaterials.

The invention provides methods for relief of stress in crosslinkedpolymers. The stress relief may be of varying degrees, and need not becomplete relief of all stress in the material. The stress may beinternal, external, or a combination thereof. Internal stress buildupduring polymerization of a crosslinked network is a typical result ofpolymerization shrinkage. Internal stress buildup may decrease theultimate mechanical properties of the cured polymer and/or limit itsapplications. For example, in polymeric coatings and dental materials,internal stress may warp or crack the material or the underlyingsubstrate.

The invention also provides monomer mixtures and polymerization methodsfor forming a crosslinked polymer network which can provide forrelaxation of internal stress during polymerization.

In an embodiment, the invention provides mixtures comprising a firstmonomer comprising an addition-fragmentation chain cleavage group in thepolymer backbone and a plurality of polymerizable groups and a secondmonomer or oligomer comprising a plurality of polymerizable groups andnot comprising an addition-fragmentation chain cleavage group. Themixture may further comprise a source of free radicals or cations. Themolar ratio of addition-fragmentation to polymerizable functional groupsof the monomer(s) may be from 0.0025 to 0.1 (i.e. 0.25% to 10%), 0.0025to 0.05 (i.e. 0.25% to 5%), 0.005 to 0.05 (i.e. 0.5% to 5%), 0.005 to0.025 (i.e. 0.5% to 2.5%), 0.01 to 0.15 (1% to 15%), 0.02 to 0.15 (2% to15%) 0.05 to 0.15 (5% to 15%), 0.075 to 0.15 (7.5% to 15%) or 0.1 to0.15 (10% to 15%). The molar ratio of the first monomer to initiator maybe from 0.05 to 10% or 1% to 5%.

In an embodiment the invention provides a mixture for forming apolymeric material comprising a crosslinked network of polymer chains,the mixture comprising

-   -   a) a first monomer comprising a reversible        addition-fragmentation chain transfer functionality in the        monomer backbone and a plurality of polymerizable groups;    -   b) a second monomer comprising a plurality of polymerizable        groups and not comprising an addition-fragmentation chain        transfer functionality; and    -   c) a source of free radicals.        wherein the addition-chain transfer functionality is a        thiocarbonylthio group. In an embodiment, the polymerizable        groups of both the first and second monomers are selected from        the group consisting of acrylate monomers and methacrylate        monomers. In an embodiment, the mixture may also comprise        components such as dyes, pigments, stabilizers, solvents and/or        rheological additives. The mixtures of the invention may be used        for forming dental materials.

In an embodiment, the first monomer is linear or aliphatic, rather thancyclic. In an embodiment, the first monomer comprises a reversibleaddition fragmentation chain cleavage group linked to a plurality ofpolymerizable end groups by alkylene (divalent hydrocarbon) or etherlinker groups. In an embodiment, the ether linker is —O-alkylene- or isformed of a plurality of —O-alkylene- groups. In an embodiment, theether linker is —OCH₂CH₂— or is formed of a plurality of —OCH₂CH₂₋units. In another embodiment, the linker group is a linear alkylenegroup which is —CH₂— or is formed of a plurality of —CH₂— repeat units.In an embodiment, the reversible addition fragmentation chain cleavagegroup is a thiocarbonylthio group. The thiocarbonylthio group may be atrithiocarbonate group or a dithioester such as an aromatic dithioester.In an embodiment, the first monomer may be as shown in Formula 21 or 22,with the polymerizable groups being acrylate or methacrylate groups. Inan embodiment, the molecular weight of the first monomer is from 400-800amu.

In an embodiment, the second monomer comprises at least two acrylate ormethacrylate end groups. The second monomer may further comprise abisphenol A-derived group or a urethane group. In an embodiment, themixture does not include an additional diluent or solvent for the secondmonomer. In an embodiment, the molecular weight of the second monomer isfrom 200-800 amu.

In an embodiment, the invention provides a dental restorativecomposition comprising an acrylate or methacrylate monomer comprising areversible addition fragmentation chain cleavage group, a methacrylatemonomer or oligomer not comprising a reversible addition fragmentationchain cleavage group, filler particles, and a free radical initiator.The amount of filler particles may be 45 to 85% by weight (wt %) of thecomposition. The filler particles may be inorganic particles or surfacetreated inorganic particles. Suitable inorganic materials particles foruse in the invention include, but are not limited to, barium containingglass, strontium containing glass, zirconia silicate and/or amorphoussilica having a size ranging from 0.005 to 5.0 micrometers or 0.01 to1.0 micrometers. In different embodiments, the methacrylate monomer oroligomer not including a reversible addition fragmentation chaincleavage group may be BIS-GMA-based or urethane methacrylate-based.

The acrylate or methacrylate monomer comprising a reversible additionfragmentation chain cleavage group may act as a reactive diluent whichmay be used to lower the viscosity and make the composition pourable atordinary room temperatures such as at about 20°-25° C. In an embodiment,the monomer(s) comprising reactive addition fragmentation chain cleavagegroups is/are the only reactive diluent(s) present in the composition.In an embodiment, the weight percentage of the monomer comprising thereversible addition fragmentation chain cleavage group may be from 1 to30 wt % of the monomer component of the composition. In anotherembodiment, the acrylate or methacrylate monomer comprising thereversible addition fragmentation chain cleavage group may be combinedwith an additional acrylate or methacrylate monomer which also acts as areactive diluent. This additional acrylate or methacrylate monomer maybe, for example, methyl methacrylate, ethyl methacrylate, n-propylmethacrylate, iso-propyl methacrylate, n-butyl methacrylate, iso-butylmethacrylate, sec-butyl methacrylate, tert-butyl methacrylate, the amylmethacrylates, ethylene dimethacrylate, butylene dimethacrylate,ethylene glycol monomethacrylate, ethylene glycol dimethacrylate,triethylene glycol dimethacrylate or tetraethylene glycoldimethacrylate.

In an embodiment, the invention provides methods for forming crosslinkedpolymeric materials. In an embodiment, the method comprises the stepsof:

-   -   a) forming a mixture comprising        -   i) a first monomer which is a linear multifunctional monomer            comprising a reversible addition-fragmentation chain            transfer functionality in the monomer backbone and a            plurality of polymerizable groups;        -   ii) a second monomer not containing an            addition-fragmentation chain transfer functionality and            containing a plurality of polymerizable groups; and        -   iii) a source of free radicals    -   b) subjecting the mixture to polymerization conditions for        sufficient time to obtain at least 50% conversion of the        polymerizable functional groups.        Polymerization of the mixture may be achieved by        photopolymerization or other means known to the art. In an        embodiment, the crosslinked polymeric materials resulting from        this process have a shrinkage stress level less than 1.5 MPa,        1.0 MPa or 0.75 MPa after polymerization to the desired extent        of conversion. In an embodiment, the glass transition        temperature of the cross-linked polymeric material is greater        than 20° C.

In an embodiment, the invention provides a method for forming a dentalrestorative material, the method comprising the steps of

-   -   a) providing a dental restorative composition comprising a        methacrylate monomer comprising a bisphenol A derived group or a        urethane group and not comprising a reversible addition        fragmentation chain cleavage group, filler particles having an        average size from 0.005 to 5.0 micrometers, a free radical        initiator and a diluent acrylate or methacrylate monomer        comprising a reversible addition fragmentation chain transfer        functionality wherein the molar ratio of addition-fragmentation        chain transfer functionalities to polymerizable groups in the        composition is from 1% to 15% and the amount of filler particles        in the composition is from 45 to 85% by weight;    -   b) subjecting the dental restorative composition to        polymerization conditions for a time sufficient to achieve        meth(acrylate) polymerizable group conversion of at least 50%        wherein the shrinkage stress level is less than 1.5 MPa        following step b). In other embodiments, dental restorative        materials resulting from this process have a shrinkage stress        level less than 1.0 MPa or 0.75 MPa. Typically, polymerization        of the mixture is induced by photopolymerization. The dental        restorative materials of the invention can have reduced        shrinkage stress levels compared to bisphenol-based restorative        materials not incorporating a monomer comprising a reversible        addition-fragmentation chain transfer functionality. For        example, by swapping a reactive diluent in a conventional dental        composite with a monomer comprising an addition-fragmentation        chain transfer functionality, the shrinkage stress may be        reduced by greater than or equal to 25% or greater than or equal        to 50%.

The methods of the invention also can provide photoinduced plasticity,actuation and equilibrium shape changes without any accompanyingresidual stress. Polymeric materials such as these that are able torespond “on demand” with control of stress, shape and plasticity arecritical to the development of microdevices, biomaterials and polymericcoatings.

The methods of the invention can achieve stress relief, photoinducedplasticity, actuation or equilibrium shape changes without anysubstantial permanent degradation in the mechanical properties of thematerial. In addition, the methods of the invention can alter thetopology of the network without permanently changing the networkconnectivity.

The invention also provides polymeric materials capable of reversiblechain cleavage and linear multifunctional monomers useful in makingthese materials. The invention further provides methods of makingpolymeric materials capable of reversible chain cleavage. Polymericmaterials made by the processes of the invention can have reduced levelsof internal stress when compared to polymeric materials made byconventional polymerization processes.

Reversible cleavage of the polymer backbone is enabled by incorporationof “reversible chain cleavage” groups or functionalities into thepolymer backbone. Under the appropriate conditions, the reversible chaincleavage groups are activated, leading to reversible cleavage of thepolymer backbone. At least some of the reversible chain cleavage groupsare incorporated mid-chain. Depending on the nature of the chaincleavage group, the chain cleavage group may be activated throughreaction with either a free radical or a cation. If the reversible chaincleavage group is an iniferter, the iniferter may be activated by lightor heat.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A: Schematic diagram of introduction of stress in a crosslinked,rubbery polymer. The original sample dimensions are shown at left. Thesample is then strained in tension and irradiated on one side at awavelength at which it is optically thick. The result is a stressgradient in the sample. At right, the sample bends as shown after theexternal tensile strain is removed.

FIG. 1B: Schematic diagram showing actuation in a crosslinked rubberypolymer. At left, the sample from FIG. 1A is irradiated on thepreviously unirradiated side. The result, shown at right, allowselimination of the introduced stress and actuation of the sample.

FIG. 2A: Stress versus time for tetrathiol/divinyl ether/MDTO materialswith four MDTO concentrations starting from constant initial strain(solid line, 0 wt %, dashed line 25 wt %, dotted line, 50 wt %,dashed-dotted line 75 wt %). The specimens were irradiated at 320-500nm, 20 mW/cm².

FIG. 2B: Stress versus time for tetrathiol/divinyl ether/MDTO materialswith four MDTO concentrations starting from constant initial stress(solid line, 0 wt %, dashed line 25 wt %, dotted line, 50 wt %,dashed-dotted line 75 wt %). The specimens were irradiated at 320-500nm, 20 mW/cm².

FIG. 3: Tetrathiol/divinyl ether/MDTO specimens with stress gradientsthrough their thickness on a 2 mm×2 mm grid. Specimens from left toright 0, 25, 50, and 75 wt % MDTO. The direction of irradiation (365 nm,20 mW/cm² for 15 seconds) used for the creation of the stress gradientwas from left to right.

FIG. 4: Three linear monomers containing addition-fragmentation chaintransfer functionalities in their backbone.

FIG. 5: Dependence of the rate of photoinduced stress relaxation forcured 75 wt % PETMP-MDTVE/25 wt % MDTO on the incident light intensity.A tensile strain of 0.060 was applied to these samples, followed byirradiation at 365 nm at various intensities (solid line, 40 mW.cm⁻²;dashed line, 10 mW.cm⁻²; dotted line, 4 mW.cm⁻²) beginning at t=0.5minutes.

FIG. 6: (A) Schematic of the allyl sulfide AFCT mechanism in thepresence of a thiyl radical which results in a symmetrical chemicalstructure that promotes reversibility. (B) Schematic of the allylsulfide AFCT mechanism in the presence of a carbon-centered radicalwhich results in an asymmetrical chemical structure and irreversibility.(C) Schematic of the RAFT mechanism of a trithiocarbonate in thepresence of a carbon-centered radical which results in a symmetricalchemical structure and complete reversibility.

FIG. 7: Materials used in Example 7: (1) BisGMA (2) TEGDMA (3) TTCDMA(4) BAPO

FIG. 8 (A) Fracture toughness, (B) Elastic modulus, and (C) tangent δ ofBisGMA-TEGDMA 70/30 wt % () and BisGMA-TTCDMA 70/30 wt % (□)composites. Composites containing 1.5 wt % BAPO, 75 wt % of silicafilter were prepared by irradiation using a 400-500 nm wavelength filterat an intensity of 70 mW/cm².

FIG. 9. Reaction behavior of the methacrylate group in BisGMA-TEGDMA70/30 wt % () and BisGMA-TTCDMA 70/30 wt % (□) composites. Resinscontain 1.5 wt % BAPO and were cured using 400-500 nm light at 70mW/cm².

FIG. 10. Polymerization shrinkage stress versus (A) time and (B)methacrylate conversion for BisGMA-TEGDMA 70/30 wt % () andBisGMA-TTCDMA 70/30 wt % (□) composites. Composites shown here contain1.5 wt % BAPO and 75 wt % silica filter, and were irradiated at 70mW/cm² using 400-500 nm light.

DETAILED DESCRIPTION

The invention provides methods for inducing reversible chain cleavage ina polymeric material comprising a crosslinked network of polymer chains.As used herein, a polymeric material comprises a polymer and may furthercomprise non-polymeric additives. In the present invention, amultiplicity or plurality of the polymer chains incorporate at least onereversible chain cleavage group in the polymer backbone. Reversiblechain cleavage groups incorporated in side chains or at the chain ends,rather than in the backbone, are not effective in producing stressrelief. Crosslinked polymer networks may be characterized by thegel-point conversion, a critical transition point where there is asingle sample spanning macromolecule. The gel-point conversion isdefined mathematically as the conversion where the weight averagemolecular weight diverges. The polymerization mechanism used to form thecrosslinked polymeric material may be any mechanism known to the art,including step-reaction (thiol-ene) and chain-reaction. In differentembodiments, the crosslinked polymeric materials of the invention mayhave at least 50%, 60%, 70%, 80% or 90% conversion of the polymerizablefunctional groups. The conversion of polymerizable functional groups maybe measured with infrared absorption spectroscopy.

As used herein, in a reversible chain cleavage process both chaincleavage and chain recombination occur. Recombination may occur throughsimple reversal of the chain cleavage reaction. In addition, theproducts of the chain cleavage reaction at one cleavage location arecapable of participating in additional chain cleavage reactions,allowing combination of chain fragments from different cleavagelocations and chain rearrangement. For example, the reversible chaincleavage process may be an addition-fragmentation process in which aradical reacts with an in-chain addition-fragmentation functionality toform an intermediate, which in turn fragments to reform the initialfunctionality and a new radical. The new radical may further react withanother in-chain functionality or the initial functionality may furtherreact with another radical generated by a differentaddition-fragmentation event. Chain rearrangement resulting from thereversible chain cleavage process can alter the topology of the network.In the absence of radical termination events or other side reactions,the number of reversible chain cleavage groups, and hence networkstrands, remains unchanged.

As used herein, a reversible chain cleavage group is capable of bothbreaking and forming in-chain bonds. In an embodiment, the reversiblechain cleavage group is a chain transfer group which undergoesaddition-fragmentation type chain transfer. As used herein,“addition-fragmentation” is either a two-step or concerted chaintransfer mechanism wherein addition of a radical or cation is followedby fragmentation to generate a new radical or cation species. Scheme 1illustrates a reaction mechanism for addition-fragmentation chaintransfer within a polymer backbone incorporating an allyl sulfidefunctionality. In Scheme 1, an allyl sulfide group in the backbonereacts with a thiyl radical generated from a previous chain cleavageevent, resulting in cleavage of the backbone and formation of adifferent thiyl radical (a R3 containing radical). The R3 containingthiol radical shown in Scheme 1 is capable of reacting with an allylsulfide group located at another chain position. The reformed allylsulfide functionality is capable of reacting with another thiyl radicalgenerated by chain cleavage.

Suitable addition-fragmentation functionalities or agents for use in theinvention include conventional reversible addition-fragmentation chaintransfer (RAFT) agents, allyl sulfides, dithiocarbamates,trithiocarbonates and thiuram disulfides. RAFT agents are known to thoseskilled in the art. Examples of RAFT agents are given in U.S. Pat. No.6,153,705, and published international applications WO 98/01478, WO99/35177, WO 99/31144 and WO 98/58974. Allylic sulfide chain transfergroups are described by Meijs et al. (1998, Macromolecules, 21(10),3122-3124). Suitable addition-fragmentation chain transfer agentsinclude trithiocarbonate or allyl sulfide functionalities.

WO 98/01478 discloses that free radical polymerizations when carried outin the presence of certain chain transfer agents of the followingstructure:

have living characteristics and provide polymers of controlled molecularweight and low polydispersity.

WO 98/01478 also discloses thiocarbonylthio compounds selected from

having a chain transfer constant greater than about 0.1 wherein Z isselected from the group consisting of hydrogen, chlorine, optionallysubstituted alkyl, optionally substituted aryl, optionally substitutedheterocyclyl, optionally substituted alkylthio, optionally substitutedalkoxycarbonyl, optionally substituted aryloxycarbonyl (—COOR_(y)),carboxy (—COOH), optionally substituted acyloxy (—O₂CR_(y)), optionallysubstituted carbamoyl (—CON(R_(y))₂), cyano (—CN), dialkyl- ordiaryl-phosphonato [—P(═O)O(R_(y))₂], dialkyl- or diaryl-phosphinato[—P(═O)(R_(y))₂], and a polymer chain formed by any mechanism, Z′ is am-valent moiety derived from a member of the group consisting ofoptionally substituted alkyl, optionally substituted aryl and a polymerchain; where the connecting moieties are selected from the group thatconsists of aliphatic carbon, aromatic carbon, and sulfur, R is selectedfrom the group consisting of optionally substituted alkyl; an optionallysubstituted saturated, unsaturated or aromatic carbocyclic orheterocyclic ring; optionally substituted alkylthio; optionallysubstituted alkoxy; optionally substituted dialkylamino; anorganometallic species; and a polymer chain prepared by anypolymerization mechanism; in the compounds of Formulas 2 and 3, R. is afree radical leaving group that initiates free radical polymerization;R_(y) is selected from the group consisting of optionally substitutedC₁-C₁₈ alkyl, C₂-C₁₈ alkenyl, aryl, heterocyclyl, aralkyl, alkarylwherein the substituents are independently selected from the group thatconsists of epoxy, hydroxy, alkoxy, acyl, acyloxy, carboxy (and salts),sulfonic acid (and salts), alkoxy- or aryloxycarbonyl, isocyanato,cyano, silyl, halo, and dialkylamino; p is 1 or an integer greater than1; when p≧2, then R═R_(x), m is an integer ≧2; and R_(x) is a p-valentmoiety derived from a member of the group consisting of optionallysubstituted alkyl, optionally substituted aryl and a polymer chain;where the connecting moieties are selected from the group consisting ofaliphatic carbon, aromatic carbon, silicon, and sulfur; in the compoundsof Formulas 2 and 3, R_(x). is a free radical leaving group thatinitiates free radical polymerization.

WO 99/31144 discloses the sulfur based CTA (chain transfer agent) offormula 4 below:

wherein when D is D1 of the following formula 5 below:

then p′ is in the range of from 1 to 200, E is Z″ and said transferagent is of the following formula 6 below:

wherein when D is D2 of the following formula 7 below:

then p′ is in the range of from 1 to 200, E is E1 or E2 and saidtransfer agent is of the following formula 8 below:

wherein when D is D3 of the following formula 9 below:

then p″ is in the range of from 2 to 200, E is Z′, E1 or E2 and saidtransfer agent is of the following formula 10 below:

orwherein when D is D4 of the following formula 11 below:

—S—R″  Formula 11

then E is E3 or E4 and said transfer agent is of the following formula12:

where in all of the above:

-   R′ is a p′-valent moiety derived from a moiety selected from the    group consisting of a substituted or unsubstituted alkane,    substituted or unsubstituted alkene, substituted or unsubstituted    arene, unsaturated or aromatic carbocyclic ring, unsaturated or    saturated heterocyclic ring, an organometallic species, and a    polymer chain, R′. being a free radical leaving group resulting from    R′ that initiates free radical polymerization; R* and R″ are    monovalent moieties independently selected from the group consisting    of a substituted or unsubstituted alkyl, substituted or    unsubstituted alkenyl, substituted or unsubstituted aryl,    unsaturated or aromatic carbocyclic ring, unsaturated or saturated    heterocyclic ring, substituted or unsubstituted alkylthio,    substituted or unsubstituted alkoxy, substituted or unsubstituted    dialkylamino, an organometallic species, and a polymer chain, R*.    being a free radical leaving group resulting from R* that initiates    free radical polymerization; X is selected from the group consisting    of a substituted or unsubstituted methine, nitrogen, and a    conjugating group; Z″ is selected from the group consisting of E1,    E2, halogen, substituted or unsubstituted alkyl, substituted or    unsubstituted alkenyl, substituted or unsubstituted aryl,    substituted or unsubstituted heterocyclyl, substituted or    unsubstituted alkylthio, substituted or unsubstituted    alkoxycarbonyl, substituted or unsubstituted —COOR′″, carboxy,    substituted or unsubstituted —CONR′″₂, cyano, —P(═O)(OR′″)₂,    —P(═O)R′″₂; R′″ is selected from the group consisting of substituted    or unsubstituted alkyl, substituted or unsubstituted alkenyl,    substituted or unsubstituted aryl, substituted or unsubstituted    heterocyclyl, substituted or unsubstituted aralkyl, substituted or    unsubstituted alkaryl, and a combination thereof; Z′″ is a p″-valent    moiety derived from a moiety selected from the group consisting of a    substituted or unsubstituted alkane, substituted or unsubstituted    alkene, substituted or unsubstituted arene, substituted or    unsubstituted heterocycle, a polymer chain, an organometallic    species, and a combination thereof; Z′ is selected from the group    consisting of a halogen, substituted or unsubstituted alkyl,    substituted or unsubstituted alkenyl, substituted or unsubstituted    aryl, substituted or unsubstituted heterocyclyl, substituted or    unsubstituted alkylthio, substituted or unsubstituted    alkoxycarbonyl, substituted or unsubstituted —COOR′″, carboxy,    substituted or unsubstituted —CONR′″₂, cyano, —P(═O)(OR′″)₂,    —P(═O)R′″₂; E1 is a substituent functionality derived from a    substituted or unsubstituted heterocycle attached via a nitrogen    atom, or is of the following Formula 13:

wherein G and J are independently selected from the group consisting ofhydrogen, substituted or unsubstituted alkyl, substituted orunsubstituted alkenyl, substituted or unsubstituted alkoxy, substitutedor unsubstituted acyl, substituted or unsubstituted aroyl, substitutedor unsubstituted aryl, substituted or unsubstituted heteroaryl,substituted or unsubstituted alkenyl, substituted or unsubstitutedalkylsulfonyl, substituted or unsubstituted alkylsulfinyl, substitutedor unsubstituted alkylphosphonyl, substituted or unsubstitutedarylsulfonyl, substituted or unsubstituted arylsulfinyl, substituted orunsubstituted arylphosphonyl; and

-   E2 is of the following formula 14:

wherein G′ is selected from the group consisting of substituted orunsubstituted alkyl, substituted or unsubstituted alkenyl, substitutedor unsubstituted aryl;

-   E3 is of the following formula 15:

wherein p″″ is between 2 and 200, G″ is Z″'and J′ is independentlyselected from the group consisting of hydrogen, substituted orunsubstituted alkyl, substituted or unsubstituted alkenyl, substitutedor unsubstituted alkoxy, substituted or unsubstituted acyl, substitutedor unsubstituted aroyl, substituted or unsubstituted aryl, substitutedor unsubstituted heteroaryl, substituted or unsubstituted alkenyl,substituted or unsubstituted alkylsulfonyl, substituted or unsubstitutedalkylsulfinyl, substituted or unsubstituted alkylphosphonyl, substitutedor unsubstituted arylsulfonyl, substituted or unsubstitutedarylsulfinyl, substituted or unsubstituted arylphosphonyl; and

-   E4 is of the following formula 16:

-   wherein p″″ is between 2 and 200 and G′ is Z′″.

U.S. Pat. No. 6,153,705 discloses compounds of the general formula

q being between 2 and 10, preferably between 2 and 5, R_(a) and R_(b),which are identical or different, represent: an optionally substitutedalkyl, acyl, aryl, alkene or alkyne group (i), or an optionallysubstituted, saturated or unsaturated, carbon-containing or aromaticring (ii), or an optionally substituted, saturated or unsaturatedheterocycle (iii), it being possible for these groups and rings (i),(ii) and (iii) to be substituted with substituted phenyl groups,substituted aromatic groups, or groups: alkoxycarbonyl oraryloxycarbonyl (—COOR_(c)), carboxy (—COOH), acyloxy (—O₂CR_(c)),carbamoyl (CON(R_(c))₂), cyano (—CN), alkylcarbonyl, alkylarylcarbonyl,arylcarbonyl, arylalkylcarbonyl, phthalimido, maleimido, succinimido,amidino, guanidimo, hydroxyl (—OH), amino (—N(R_(c))₂), halogen, allyl,epoxy, alkoxy (—OR_(c)), S-alkyl, S-aryl, groups having a hydrophilic orionic character, such as the alkali metal salts of carboxylic acids, thealkali metal salts of sulphonic acid, polyalkylene oxide chains (PEO,PPO), cationic substituents (quaternary ammonium salts), R_(c)representing an alkyl or aryl group, a polymer chain, Z_(a) is S or P,Z_(b) is O, S or P.

Each reversible addition-fragmentation chain transfer agent is activatedby reaction with a free radical or cation, the reaction being additionof the free radical or cation to the chain transfer agent. Thereversible chain cleavage process is initiated by activating a source offree radicals or cations (other than the addition -fragmentation chaintransfer agent); this source may also be termed a reversible chaincleavage initiator. At a later stage in the process, radicals or cationscapable of activating the reversible addition-fragmentation chaintransfer agents are also generated during the fragmentation steps. Toobtain the desired amount of reversible chain cleavage, it may bedesirable to continue to activate the source of free radicals or cationsafter the start of the reversible chain cleavage process. The recurringchain transfer reactions amplify the effects of each generated radicaland lead to reaction diffusion of radicals through the cross-linkedmatrix.

When the reversible chain cleavage group is an addition-fragmentationgroup, a source of free radicals or cations is present during thepolymerization process and is optionally incorporated into the polymericmaterial. A source of free radicals or cations is incorporated into thepolymeric material if the reversible chain cleavage groups are to beactivated after polymerization.

In an embodiment, the source of free radicals is a photoinitiator or athermally activated initiator. In another embodiment, the source of freeradicals is an iniferter, which may or may not be incorporated in thepolymer chains. Use of an iniferter as the source of free radicals isexpected to provide a virtually endless source of radicals, sinceiniferters are expected to be depleted significantly more slowly thanmore traditional radical sources. In another embodiment the source offree radicals is a redox system. In another embodiment, the source ofcations is a photoacid generator such as a diaryliodonium or triarylsulfonium salts. As used herein, the source of free radicals or cationsis activated when it produces free radicals or cations. In anembodiment, the source of radicals or cations is activated so as toproduce a greater concentration of radicals or cations than would beproduced by ambient conditions. For example, a photoinitiator oriniferter may be activated by an artificial light source such as a UVlamp rather than by simple exposure to sunlight. The source of freeradicals or cations may consist of a combination of different sources offree radicals or cations. In an embodiment, the source of free radicalscomprises two photoinitiators which are activated by differentwavelengths of light. In this embodiment, one photoinitiator may beactivated during the polymerization process and the other activatedsubsequent to polymerization. For example, one photoinitiator may be aphotoinitiator active in the visible region of the spectrum and theother may be active in the near ultraviolet.

The source of free radicals or cations may be activated by any methodknown to the art, including exposure to light, heat, or an electronbeam. The time for which the free radical/cation source is activated andthe concentration of the free radical/cation source can influence theamount of backbone cleavage which occurs. The source of free radicals orcations can be activated during polymerization, followingpolymerization, or both.

The reversible chain cleavage initiators are present in an amounteffective to produce the desired amount of reversible chain cleavage. Inone embodiment where the polymeric material is made by polymerization ofa mixture comprising at least one monomer containing an in-chainreversible chain cleavage group, the amount of reversible chaininitiator provided in the mixture is greater than the amount that wouldtypically be used for polymerization of a similar mixture comprising asimilar monomer not containing an in-chain reversible chain cleavagegroup. As described above a combination of initiators may also be usedto ensure that sufficient initiator remains after polymerization iscomplete. The effective amount of reversible chain initiator will dependon the efficiency of the system, the time for during which reverse chaincleavage occurs, and the amount of impurities in the system. In anembodiment, the concentration of the source of free radicals or cationsis between 0.1 wt % and 5.0 wt %.

In another embodiment, the reversible chain cleavage group is a reactivegroup which undergoes reversible fragmentation and recombination. Chaincleavage occurs during the fragmentation step. In this embodiment, thereversible chain cleavage group is activated by exposure to conditionswhich initiate fragmentation of the reversible chain cleavage group. Noseparate source of free radicals or cations (other than the reversiblechain cleavage group) is required. Suitable reactive groups useful forthis embodiment include iniferters. Iniferter compounds are known tothose skilled in the art and include, but are not limited to,dithiocarbamates and thiuram disulfides. Iniferters contain a chemicalbond that will homolytically cleave under appropriate thermal orphotolytic conditions, forming either one sulfur centered and one carboncentered radical or two sulfur centered radicals. Rearrangement of thenetwork connectivity occurs when radicals generated by two differentchain cleavage events recombine. In this embodiment, during the cleavageevent the crosslink density can temporarily decrease, but returns tonormal after recombination.

In another embodiment, the polymeric material incorporates a pluralityof chemically different reversible chain cleavage groups. For example,the polymeric material may incorporate two chemically differentreversible addition-fragmentation chain transfer groups or mayincorporate at least one reversible addition-fragmentation chaintransfer group and at least one fragmentation and recombination group.For example, including an addition-fragmentation functionality incombination with an iniferter as a source of free radicals has thepotential to allow the production of unlimited plasticity in thematerial which wouldn't be possible with a conventional initiator. Aconventional initiator would be expected to run out after a certainlight dose (for a photoinitiator) or time (for a thermal initiator).

In an embodiment, the reversible chain cleavage group can beincorporated into the polymer backbone by polymerization of a monomercomprising the chain cleavage group. The monomer comprising the chaincleavage group may be a ring opening monomer or a linear monomer. Ringopening monomers suitable for the practice of the invention include, butare not limited to, cyclic allylic sulfides. Cyclic allylic sulfidemonomers capable of free-radical ring-opening polymerization arereported by Evans and Rizzardo (Evans, R. et al., 2001, J. Polym. Sci.A, 39, 202) and in U.S. Pat. No. 6,043,361 to Evans et al.

Linear monomers suitable for the practice of the invention includemultifunctional monomers containing an addition-fragmentation chaintransfer functionality in their backbone, as shown in Formula 20.

A₁-Y-A₂   Formula 20

In Formula 20, A₁ and A₂ each independently comprise at least onepolymerizable group. Examples of polymerizable groups include, but arenot limited to, acrylates, methacrylates, oxiranes(epoxies), amines,vinyl ethers, vinylics, allyl ethers, allylics, thiols, styrenics,isocyanates, and hydroxides, and combinations thereof. In an embodiment,the polymerizable group is selected from the group consisting ofacrylates, methacrylates, vinyl ethers, and allyl ethers. Y is areversible chain cleavage group. A₁, A₂ and Y may be attached togetherwith any suitable linker group such as a C₁-C₁₀ alkyl chain that may beoptionally substituted with cyclic or aromatic functionalities or suchas an ether. In an embodiment, the multifunctional monomer is adifunctional monomer. These linear monomers may be made by syntheticmethods known to those skilled in the art.

In an embodiment, the difunctional monomer contains a trithiocarbonateaddition-fragmentation chain transfer functionality in its backbone.Each of the in-chain sulfur atoms may be bonded to a tertiary carbon,which is in turn bonded to a linking group, the linking group beingbonded to a polymerizable group (e.g. the polymerizable groups (PGs) areend groups). Such a difunctional monomer may be described by Formula 21or 22

where L1-L4 are linking groups and PG1 and PG2 are polymerizable groups.In an embodiment, R4, R5, R6, and R7, independently, can be the same ordifferent and can be a linear or branched alkyl group having from 1 to 6carbon atoms, a linear or branched alkyl group having from 1 to 6 carbonatoms and substituted with an aryl group or substituted aryl group or anaryl group or a substituted aryl group, having from 1 to 6 substituentson the aryl ring. In an embodiment, the substituent on the aryl ring maybe an alkyl group having from one to 6 atoms, fluorine, chlorine, acyano group, or an ether having a total or 2 to 20 carbon atoms. In anembodiment, R4, R5, R6, and R7 are methyl or phenyl. In formula 21, L₁and L₂, independently, can be the same or different and can be an ethergroup or an alkylene group. The ether linker may have 1-12, 2-10, 4-8 or6 carbon atoms. In an embodiment, the linking group is —(CH₂—CH₂₋O)_(g)—where g is an integer from 1 to 6 (where an oxygen atom in the repeatunit is bound to the carbonyl carbon). In an embodiment, L₁ is —CH₂CH₂O—or formed of a plurality of —CH₂CH₂O— repeat units and L₂ is —OCH₂CH₂—or formed of a plurality of —OCH₂CH₂— repeat units (where an oxygen atomin the repeat unit is bound to the carbonyl carbon). In formula 22, L₃and L₄, independently, can be the same or different and can be an ethergroup or an alkylene group. In formulas 21 and 22 PG1 and PG2 can be thesame or different and can be acrylate CH₂═CHCOO— or methacrylateCH₂═C(CH₃)COO—.

In an embodiment, the linear multifunctional monomer is as shown inFormula 23:

In an embodiment, A1 and A2 comprise polymerizable groups which are bothacrylate, methacrylate, or vinyl ether groups. In Formula 23, Y offormula 20 is an allyl disulfide functionality.

Use of linear monomers rather than cyclic ring-opening monomers in thepractice of the invention can have several advantages. The use of linearmonomers allows much greater flexibility as to potential monomerstructure and can allow for increased glass transition temperatures. Themonomer synthesis process can also be simplified. The polymerizationreaction can also be much faster and higher crosslink densities can beachieved, allowing attainment of higher glass transition temperatures.Materials with higher transition temperatures are more useful forstructural purposes which require a hard glassy material rather than asoft rubbery material.

In another embodiment, the reversible chain cleavage group can beincorporated into the polymer backbone by copolymerization of a firstmonomer comprising the reversible chain cleavage group with at least oneother monomer. The other monomer(s) to be copolymerized with the firstmonomer can be any monomer known to the art, so long as it can besuccessfully copolymerized with the first monomer. The amount of thefirst monomer is selected to provide the desired amount of reversiblechain cleavage group incorporation into the polymer backbone. Theselection of the amount of the first monomer depends in part on thedesired amount of stress relief and the desired mechanical or chemicalproperties of the copolymer.

Monomers suitable for the practice of the invention include those whichare capable of forming cross-linked networks of polymer chains, eithersingly or in combination with one or more other monomers. In anembodiment, suitable monomers for the practice of the invention include,but are not limited to: ethylene oxides (for example, PEO), ethyleneglycols (for example, PEG), vinyl acetates (for example, PVA), vinylpyrrolidones (for example, PVP), ethyloxazolines (for example, PEOX),amino acids, saccharides, proteins, anhydrides, vinyl ethers, thiols,amides, carbonates, phenylene oxides (for example, PPO), acetals,sulfones, phenylene sulfides (for example, PPS), esters, fluoropolymers,imides, amide-imides, etherimides, ionomers, aryletherketones, olefins,styrenes, vinyl chlorides, ethylenes, acrylates, methacrylates, amines,phenols, acids, nitriles, acrylamides, maleates, benzenes, epoxies,cinnamates, azoles, silanes, chlorides, epoxides, lactones, isocyanates,hydroxides and amides.

In an embodiment, a monomer with acrylate or methacrylate polymerizablegroups may also comprise at least one bisphenol A-derived group. As usedherein, a monomer including a bisphenol A-derived group includes thegroup:

Suitable methacrylate monomers with bisphenol A-derived “cores” includeBisphenol A ethoxylate dimethacrylate (BisEMA) and bisphenol A glycidylmethacrylate (BisGMA), bisphenol A dimethacrylate (bis-DMA) Suitableacrylate monomers with bisphenol A-derived cores include Bisphenol Aethoxylate diacrylate. In an embodiment, the monomer is Bis-GMA basedmonomer or oligomer represented by the Formula 24

where f is from 1-10, 1-5, or 1-3. In another embodiment, the Bis-GMAbased monomer may be a modified bis-GMA type monomer or oligomer. Forexample, the OH groups of a bis-GMA type monomer may be reacted withalkyl isocyanates as described by Khatri et al. ACS Polymer Preprints 41(2). 1724-1725, hereby incorporated by reference.

In an embodiment, a monomer with acrylate or methacrylate polymerizablegroups may also comprise at least one urethane group. In an embodiment,the monomer comprises from 2-4 or 2-6 urethane groups. Hydrogen bondingbetween urethane groups can enhance the toughness of the polymer. Suchmonomers can be aliphatic or aromatic. Examples ofacrylates/methacrylates with urethane groups include, but are notlimited to7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-diylbis(2-methylacrylate) and2-((((5-(((2-(methacryloyloxy)ethoxy)carbonyl)amino)-1,3,3-trimethylcyclohexyl)methyl)carbamoyl)oxy)ethylmethacrylate. In an embodiment, the urethane dimethacrylate has theformula:

The amount of the monomer(s) containing the addition-fragmentationfunctionality may be referenced to the concentration of the monomer notincluding the addition-fragmentation functionality, the concentration ofpolymerizable functional groups or given as its concentration in themonomer mixture. The molar ratio of the addition-fragmentation groups topolymerizable functional groups of the monomer(s) may be from 0.0025 to0.1 (i.e. 0.25% to 10%), 0.0025 to 0.05 (i.e. 0.25% to 5%), 0.005 to0.05 (i.e. 0.5% to 5%), or 0.005 to 0.025 (i.e. 0.5% to 2.5%), 0.01 to0.15 (1% to 15%), 0.02 to 0.15 (2% to 15%) 0.05 to 0.15 (5% to 15%),0.075 to 0.15 (7.5% to 15%) or 0.1 to 0.15 (10% to 15%). Alternately,the weight percentage of RAFT monomer in the monomer solution may befrom 1 to 10 wt %.

The compositions, monomer mixtures and crosslinked polymeric materialsof the invention may also comprise filler particles. A variety ofinorganic filler materials can be used, including, but not limited to,silica, quartz, glass, silicates such as calcium silicate and zirconiumsilicate, metal oxide powders such as aluminum or zinc oxide and calciumcarbonate. These fillers may be surface treated and for example can haveorganosilyl groups on their surface. For dental restorative materials,the filler particles may be silanized filler compounds such as bariumcontaining glass, strontium containing glass, zirconia silicate and/oramorphous silica to match the color and opacity to a particular use ortooth. The filler is typically in the form of particles with a sizeranging from 0.01 to 5.0 micrometers. The amount of filler particles maybe 45 to 85% by weight (wt %) of the monomer mixture.

One measure of the relative number of reversible chain cleavage groupsin the crosslinked polymer network is the ratio of the number ofreversible chain transfer groups to the number of crosslinks in thenetwork. This ratio is selected based partly on the desired extent ofreversible chain cleavage in the network and partly on the desiredmechanical properties of the material (e.g. the elastic modulus). Indifferent embodiments, the ratio of the number of reversible chaintransfer groups to the number of crosslinks in the network is greaterthan or equal to one-quarter, greater than or equal to one-half, greaterthan or equal to one, greater than or equal to one and one-half, orgreater than or equal to two.

The invention also provides methods for making polymeric materials. Inan embodiment, the method comprises the steps of forming a mixturecomprising one or more monomers, wherein at least one monomer ischaracterized by Formulas 20, 21, 22 or 23 and subjecting the mixture topolymerization conditions. If the reversible chain cleavage group is anaddition-fragmentation group, the mixture further comprises a source offree radicals or cations. As used herein, polymerization conditions arethose conditions that cause the at least one monomer to form a polymer,such as temperature, pressure, atmosphere, ratio of starting componentsused in the polymerization mixture, reaction time, or external stimuliof the polymerization mixture. The polymerization process can be carriedout in bulk, or solution, or other conventional polymerization modes.The process is operated at any of the reaction conditions appropriate tothe polymerization mechanism.

The polymerization mechanism may be any mechanism known to the art,including step-reaction and chain-reaction. When the reversible chaincleavage agent is an addition-fragmentation chain transfer agent, thepolymerization process is conducted in the presence of a freeradical/cation source to ensure that the free radical/cation source isincorporated in the polymeric material. However, depending on the natureof the polymerization process, the free radical/cation source may or notbe activated during polymerization. In an embodiment, the method formaking the polymeric material forms a mixture of a monomer described byFormula 20, wherein Y is an addition-fragmentation agent activated byfree radicals, and a source of free radicals, but the source of freeradicals is not activated as part of the polymerization conditions. Thecombination of the monomer(s) and the free radical/cation source may beselected to produce reversible chain cleavage during the polymerizationprocess. For example, if the polymerization mechanism involvesfree-radical polymerization an addition-fragmentation chain transfergroup such as an allyl sulfide is likely to lead to chain transferduring the polymerization process. If chain transfer occurs duringpolymerization, the reversible chain-cleavage group incorporated intothe backbone may differ somewhat from the reversible chain-cleavagegroup present in the monomer (e.g. the R groups may change).

The combination of the monomer and the free radical/cation source mayalso be selected so that reversible chain cleavage does not occur duringthe polymerization process, in which case the free radical/cation sourceis present during polymerization to ensure its incorporation into thepolymeric material. If the polymerization mechanism does not involvefree radicals, an addition-fragmentation chain transfer functionalitysuch as an allyl sulfide will not usually lead to chain transfer duringthe polymerization process. For example, divinyl ether functionalizedmonomers containing an addition-fragmentation chain transfer group canbe cationically photopolymerized at wavelengths different than thosewhich activate a photoinitiator free radical source. Similarly, epoxyamine systems in which at least one monomer incorporates a reversiblechain cleavage group can be polymerized without activating thereversible chain cleavage group. However, chain transfer can be “forced”in this type of system by activating a source of radicals or cationsthat is not required for the curing process (e.g. by activating aphotoinitiator during the curing process).

If reversible chain cleavage is to occur after polymerization, thepolymerized material is exposed to conditions sufficient to causeactivation of the reversible chain cleavage group. If the reversiblechain cleavage group is an addition-fragmentation chain transfer group,polymerized material is also exposed to conditions sufficient to causeactivation of the source of free radicals or cations. The free radicalsor cations produced can react with the reversible chain-cleavage groups,thereby activating the reversible chain-cleavage groups. Reaction of achain transfer group with the free radical or cation produced by thefree radical/cation source can result in generation of a free radicaldifferent than that produced by the free radical/cation source; thisnewly generated free radical can react with other reversible chaintransfer groups in turn.

The end result of the polymerization process is a polymeric materialcomprising a crosslinked network of polymer chains. In an embodiment,the present invention provides polymeric materials capable of reversiblechain cleavage subsequent to polymerization. In the polymeric materialsof the invention, a plurality or multiplicity of the polymer chainsincorporate at least one reversible chain cleavage group in the polymerbackbone. If the reversible chain cleavage group is anaddition-fragmentation chain transfer group, the polymeric materialfurther comprises an effective amount of a free radical/cation source.The novel crosslinked polymeric materials of the invention are notintended to include previously reported polymeric materials whichcontain in-chain allyl sulfide groups or thiuram disulfide groups, suchas the materials reported by Evans and Rizzardo (Evans, R. et al., 2001,J. Polym. Sci. A, 39, 202) and in U.S. Pat. No. 6,043,361 to Evans etal. or the thiuram disulfide containing materials reported by Clouet inU.S. Pat. No. 5,658,986 and publication WO 91/05779. Some of thesepreviously reported materials may include residual levels of thermalinitiators or photoinitiators following polymerization.

When stress relief occurs during polymerization, the invention canprovide methods of making crosslinked polymeric materials with lowershrinkage stress than materials made without the use of reversibleaddition-fragmentation monomers. In an embodiment, the crosslinkedpolymeric materials resulting from this process have a shrinkage stresslevel less than 1.5 MPa, 1.0 MPa, or 0.75 MPa.

The invention also provides methods of making crosslinked polymericmaterials capable of reversible chain cleavage. In an embodiment, themethods involve polymerizing a monomer with a mid-chainaddition-fragmentation group. However, the polymerization conditions mayvary. For example, polymerization may take place in the presence of asource of free radicals or cations although the polymerization does notoccur through free radical polymerization. In addition, thepolymerization may involve the monomers of Formulas 20, 21, 22 or 23.

The invention provides methods for relief or reduction of stress incrosslinked polymers. The stress relief may be of varying degrees, andneed not be complete relief of all stress in the material. For example,the amount of stress reduction can range from less than one percent tonearly one hundred percent. The stress may be internal, external, or acombination thereof. In an embodiment, the methods of the invention canalso be used to produce materials which have lower internal stress thanwould be produced with polymerization techniques previously known to theart.

The amount of stress relaxation depends upon the number of reversiblechain cleavage groups in the material, the amount of time during whichreversible chain cleavage occurs, and the conditions under whichreversible chain cleavage occurs (e.g. when reversible chain cleavage isinitiated by a photoinitiator, the intensity of the light source). For agiven material under a given initial stress, the stress level in thematerial can decrease more rapidly at the start of the stress relaxationprocess and then level out at longer times. One measure of the amount ofstress relief is the percent change in stress in a material having aconstant initial stress level, either at a set time or after the stresslevel has started to level out. In this embodiment, the reduction instress obtained after 5 minutes can be greater than or equal to 10%,greater than or equal to 20%, or greater than or equal to 30%. Thereduction in stress obtained at longer times can be greater than orequal to 90%. Another indicator of the amount of stress relief is theamount of creep, or the amount of deformation which occurs underconstant load, due to the stress relaxation process. A third indicatorof the amount of stress relief is the percent change in the stress levelin a material under constant strain. In this embodiment, the reductionin stress obtained after 5 minutes can be greater than or equal to 20%,greater than or equal to 30%, or greater than or equal to 40%. Thereduction in stress obtained at longer times can be greater than orequal to 90%. The reduction in stress as a function of time can beobtained through mechanical testing techniques known to those skilled inthe art. In some cases, such as for rubbery samples, the stress can“even out” over more than the irradiated portion of the sample. Formaterials which display visco-elastic behavior the strain rate should beselected appropriately.

Reversible chain cleavage within the polymeric material can reduce thelevel of internal stress in the material. Internal stress buildup duringpolymerization of a crosslinked network is a typical result ofpolymerization shrinkage. As used herein, internal stress is the stressthat exists in a solid when no force is applied.

Reversible chain cleavage within the polymeric material afterpolymerization can also allow actuation of the material in response tolight. In this case, either the reversible chain cleavage group or thereversible chain cleavage initiator is light activated. In anembodiment, a stress is evenly placed throughout an optically thicksample followed by irradiation of only one side of the sample so thatradicals are produced and the stress is relieved preferentially on theirradiated side. A stress gradient is therefore introduced through thesample thickness (see FIG. 1 a) which, upon removal from the apparatus,results in the sample bending to distribute the stress evenly throughoutthe sample. Actuation in the stressed sample is possible by relievingthe induced stress on a single side, as the sample deforms continuouslyto eliminate the subsequent stress gradient (FIG. 1 b). As shown in FIG.1 b, if the introduced stress is completely eliminated the sample canreturn to its original flat shape (although the length of the sample,d2, is larger than that of the sample in its initial state). Therefore,the polymeric material of the invention, when irradiated appropriately,can display a shape memory effect.

Photoinduced actuation has several advantages over more commonthermally-induced shape memory. The lack of a need for a temperaturerise to instigate a phase change leading to actuation is important innumerous thermally sensitive applications. Additionally, photopatterningmay be utilized in both the formation and release of the temporaryshape, thus actuation patterns may be achieved which are unobtainablevia thermal methods. For example, the sample can be masked, strained intension and irradiated on both sides in an alternating pattern. Theresultant shape may be released in any chosen arbitrary pattern, givenappropriately designed and patterned light exposure.

Photoinduced actuation via this addition-relaxation chain transferprocess is able to avoid several of the limitations posed by thepreviously described photoinduced actuation methods ((Eisenbach, C. D.,1980, Polymer 21(10), 1175-1179) (Finkelmann, H. et al., 2001, Phys.Rev. Lett 87(1), 15501-15504) (Li, M.-H. et al., 2003, Adv. Mater.15(7-8), 569-572) (Ikeda, T. et al., 2003, Adv. Mater. 15(3),201-205(Athanassiou, A., et al. 2005, Adv. Mater. 17(8), 988-992)(Lendlein, A. et al., 2005, Nature 434, 879-882)). Every other systemdescribed in these references exhibits a photon to reaction event rationo greater than one, where one photon either causes onephotoisomerization reaction or one bond cleavage. Conversely, while onephoton in the process described here leads to at most a single cleavageevent of a photoinitiator molecule, the two radicals formed as a resultof the photoinitiator cleavage can lead to many chain transfer events,i.e. network strand breakage and reformation. Additionally, a largerange of photoinitiating systems with photoactivities ranging from theultraviolet to the near infrared are available (Fouassier, J.-P., 1995,Photoinitiation, Photopolymerization, and Photocuring: fundamentals andapplications, Hanser, Munich), thus inherent limitations imposed on theusable wavelengths for other systems are readily avoidable.

In one embodiment which allows rapid photoinduced actuation via stressrelaxation, the network has the following characteristics: the networkhas a glass transition temperature (T_(g)) below room temperature (soadequate chain mobility exists at working temperatures), a sufficientlyhigh crosslink density (and hence a sufficiently high rubbery modulus),a sufficiently high ability to be elastically strained in tensionwithout breaking (a sufficiently high breaking strain), and shouldrespond rapidly and extensively to relieve stress in only a portion ofthe network. The combination of high modulus, high breaking strain andthe ability to relieve stress in only one part of the polymer materialenables the generation of a large amount of internal stress in thematerial.

It should also be noted that the actuation produced by this system isnot limited to photoactivation. Other stimuli able to produce radicalspreferentially at one surface, such as chemical species able to induceredox reactions or temperature to induce decomposition of thermalinitiators, are capable of producing actuation. While perhaps lesscontrollable than irradiation exposure, these mechanisms increase theflexibility of the current system, allowing it to be applied to a hostof applications.

In an embodiment, the methods for relief of stress involve making apolymeric material capable of reversible chain cleavage by the methodsof the invention and then inducing reversible chain cleavage in thematerial.

In an embodiment, the invention provides a method for relieving stressin a crosslinked polymeric material, the method comprising the steps of:

-   -   a) providing a monomer comprising a reversible        addition-fragmentation group;    -   b) polymerizing the monomer in the presence of a source of free        radicals or cations, the polymerization occurring under        conditions sufficient to form a polymeric material comprising        the source of free radicals or cations and a crosslinked network        of polymer chains, wherein a multiplicity of the polymer chains        incorporate at least one reversible chain cleavage group in the        polymer backbone; and    -   c) after polymerization of the monomer, providing conditions        sufficient to activate the source of free radicals or cations        and the reversible chain cleavage groups, thereby causing        polymer backbone chain cleavage and relief of stress in the        polymeric material.

In another embodiment, the invention provides a method for relievingstress in a crosslinked polymeric material, the method comprising thesteps of:

-   -   a) providing at least two monomers, at least one of the monomers        comprising a reversible addition-fragmentation group;    -   b) co-polymerizing the monomers in the presence of a source of        free radicals or cations, the co-polymerization occurring under        conditions sufficient to form a polymeric material comprising        the source of free radicals or cations and a crosslinked network        of polymer chains, wherein a multiplicity of the polymer chains        incorporate at least one reversible chain cleavage group in the        polymer backbone; and    -   c) after polymerization of the monomers, providing conditions        sufficient to activate the source of free radicals or cations        and the reversible chain cleavage groups, thereby causing        polymer backbone chain cleavage and relief of stress in the        polymeric material.

The invention also provides similar methods for relieving stress usingreversible fragmentation and recombination chain cleavage groups, exceptthat a separate source of free radicals or cations is not required.

The stress relief provided by the methods of the invention enables thematerial to be “molded” and subsequently destressed, allowing forarbitrary shapes to be obtained after cure.

The methods of the invention can be used to selectively relieve stressor to introduce stress gradients in a cross-linked material. Selectiveactuation of the source of free radicals or cations in a portion of thematerial can lead to relief of stress in that portion only. For example,irradiation of one side of an optically thick material under stress cancause relief of stress in the irradiated portion only, which releasesstress preferentially on the exposed side. The ability to selectivelyrelieve stress in different portions of the material also allowsshape-change or actuation phenomena. For example, irradiation of oneside of a thin specimen of optically thick material can cause curvatureof specimen, with the specimen warping away from the direction ofirradiation. Irradiation of the other side of the specimen releases thecurvature. As used herein, in an optically thick material the lightintensity is significantly attenuated as the light passes through thematerial. In an embodiment, the attenuation is at least 10%.

The invention therefore also provides methods for photoactuation ofpolymeric materials capable of reversible chain cleavage. In anembodiment, a photoactuation method includes the steps of introducing aninternal stress gradient into an optically thick material capable ofreversible chain cleavage and then releasing at least part of theinternal stress by irradiating the material. The internal stressgradient may be introduced into the material part by placing it underexternal tensile stress, irradiating part of the material, and thenreleasing the external stress.

Every formulation or combination of components described or exemplifiedcan be used to practice the invention, unless otherwise stated. Specificnames of compounds are intended to be exemplary, as it is known that oneof ordinary skill in the art can name the same compounds differently.When a compound is described herein such that a particular isomer orenantiomer of the compound is not specified, for example, in a formulaor in a chemical name, that description is intended to include eachisomers and enantiomer of the compound described individual or in anycombination. When a group of substituents is disclosed herein, it isunderstood that all individual members of those groups and allsubgroups, including any isomers and enantiomers of the group members,and classes of compounds that can be formed using the substituents aredisclosed separately. When a Markush group or other grouping is usedherein, all individual members of the group and all combinations andsubcombinations possible of the group are intended to be individuallyincluded in the disclosure.

One of ordinary skill in the art will appreciate that methods, startingmaterials, and synthetic methods, other than those specificallyexemplified can be employed in the practice of the invention withoutresort to undue experimentation. All art-known functional equivalents,of any such methods, starting materials, and synthetic methods, areintended to be included in this invention. Whenever a range is given inthe specification, for example, a temperature range, a time range, or acomposition range, all intermediate ranges and subranges, as well as allindividual values included in the ranges given are intended to beincluded in the disclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention as defined by the appended claims.

In general the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. Any precedingdefinitions are provided to clarify their specific use in the context ofthe invention.

All patents and publications mentioned in the specification areindicative of the levels of skill of those skilled in the art to whichthe invention pertains. All references cited herein are herebyincorporated by reference to the extent that there is no inconsistencywith the disclosure of this specification. Some references providedherein are incorporated by reference herein to provide detailsconcerning additional starting materials, additional methods ofsynthesis, additional methods of analysis and additional uses of theinvention.

EXAMPLE 1 Formulation of Tetrathiol/Divinyl Ether/MDTO Materials

The base network was formed from a stoichiometric mixture ofpentaerythritol tetra(3-mercaptopropionate) (PETMP) andtriethyleneglycol divinylether (TEGDVE) which produces a rubbery networkwith a glass transition temperature of approximately −25° C. Thismonomer system was modified with the addition of varying concentrationsof the ring opening monomer 2-methyl-7-methylene-1,5-dithiacyclooctane(MDTO) as a comonomer.

Resins were formulated with two photoinitiators, one active in thevisible region of the spectrum (Irgacure 784 (a titanocene derivativefrom Ciba Specialty Chemicals)) at 0.1 wt % and one active in the nearultraviolet (Irgacure 819 (a phenylphosphine oxide derivative fromCiba)) at 0.25 wt %, unless otherwise stated. The visible photoinitiatorphotolyses rapidly and photobleaches significantly during the curingstage, thus the ultraviolet photoinitiator was included in theformulation to ensure residual photoinitiator capable of introducingradicals in each specimen upon irradiation. Each resin consisted of astoichiometric ratio of PETMP and TEGDVE as well as varyingconcentrations of MDTO.

EXAMPLE 2 Stress Relaxation of Tetrathiol/Divinyl Ether/MDTO Materials

Specimens for stress relaxation experiments had approximate dimensionsof 60 mm long×7 mm wide×0.9 mm thick. These specimens were mounted in aMTS Synergie 100 mechanical tester fitted with a 10 N load cell and theinitial gap between the grips was set to 40 mm. For the ‘constantinitial stress’ experiments, a strain rate of 0.0064 min⁻¹ was applieduntil a stress of 0.15 MPa was achieved then the strain was maintainedfor the remainder of the experiment. The specimen was then irradiated(320-500 nm, 20 mW·cm⁻²) for 15 minutes, starting 60 seconds after thestress of 0.15 MPa was achieved.

FIGS. 2A and 2B show stress versus time for four MDTO concentrations(solid line, 0 wt %, dashed line, 25 wt %, dotted line, 50 wt %,dashed-dotted line, 75 wt %). FIG. 2A is for constant strain(irradiation started at t=330 sec); FIG. 2B is for constant initialstress (offset to align the start of irradiation at t=0, irradiationstopped at 900 sec). .At a constant applied stress, both the rate anddegree of stress relaxation increase with MDTO concentration. Withoutwishing to be bound by any particular theory, we attribute this responseto an increased probability of addition-fragmentation chain transfergroups in the network strands. After cessation of irradiation, thestress actually rose slightly due to shrinkage of the specimen due to acooling effect when the heating by the lamp is terminated.

Results presented in FIGS. 2A and 2B would appear similar if themechanism of stress relaxation was not due to chain transfer but ratherwas the result of a decrease in the modulus due to photodegradation. Itis possible to determine whether the network is simply undergoingphotodegradation during irradiation, perhaps by the shorter wavelengthultraviolet, by measuring the modulus of the material both before andafter irradiation. The elastic moduli (determined in tension) of thespecimens both before and after the irradiation experiments shown inFIG. 2A are presented in Table 1. The slight increase in the moduluspost-irradiation clearly indicates that photodegradation is notresponsible for the dramatic stress relaxation.

The ratio of cross-links to allyl sulfide groups was as follows:

0 wt % MDTO 1:0, 25 wt % MDTO 1.17:1, 50 wt % MDTO 0.390:1, 75 wt % MDTO0.130:1.

TABLE 1 [MDTO] Modulus before extension Modulus after extension and (wt%) and irradiation (MPa) irradiation (MPa) 0 11.5 11.8 25 7.33 7.72 504.58 5.17 75 2.38 2.92

EXAMPLE 3 Selective Stress Relief of Tetrathiol/Divinyl Ether/MDTOMaterials

Stress gradients through specimens were introduced by irradiatingrectangular, optically thick specimens on one side only that were undertension. These specimens were almost equivalent to those used in thestress relaxation experiments; however, they contained 0.1 wt % Irgacure784 and 2.5 wt % Irgacure 819 before polymerization to yield asignificant optical gradient through the specimen thickness. They wereirradiated at 365 nm, 20 mW·cm⁻² for 15 seconds while under a strain of0.0375.

FIG. 3 shows specimens with stress gradients through their thickness ona 2 mm×2 mm grid. Specimens from left to right: 0, 25, 50 and 75 wt %MDTO. The direction of irradiation (365 nm, 20 mW·cm⁻² for 15 seconds)used for the creation of the stress gradient for each specimen was fromleft to right.

EXAMPLE 4 Linear Difunctional Monomers Containing Addition-FragmentationChain Transfer Functionalities in Their Backbone

FIG. 4 illustrates three linear monomers containingaddition-fragmentation chain transfer functionalities in their backbone,these monomers include difunctional acrylate and methacrylate monomers,as well as a linear divinyl ether monomer. Synthesis methods for themonomers shown in FIG. 4 are given below.

Synthesis of a Dimethacrylate Containing an Addition-FragmentationFunctionality (2-methylene-propane-1,3-di(thioethyl methacrylate)(MDTMA)) Synthesis of2-[2-(2-hydroxy-ethylsulfanylmethyl)-allylsulfanyl]-ethanol:3-Mercapto-2-(mercaptomethyl)-1-propene was synthesized according to theliterature method (Evans, R. A. and Rizzardo, E. 2000, Macromolecules,33(18), 6722-6731. Sodium (8.05 g, 0.35 mol) was dissolved in drymethanol under nitrogen and refluxed. Solutions of3-mercapto-2-(mercaptomethyl)-1-propene (18.35 g, 0.15 mol) in 30 mlmethanol and 2-chloroethanol (27.20 g, 0.34 mol) in 30 ml methanol wereadded simultaneously to the methanolic sodium solution and allowed toreflux for three hours. The methanol was then evaporated off, water wasadded and the solution was extracted with ethyl acteate. The ethylacetate extract was washed with water, dried over calcium chloride,filtered and evaporated to yield 26.21 g (66% yield). Under nitrogen,2-[2-(2-hydroxy-ethylsulfanylmethyl)-allylsulfanyl]-ethanol (12.98 g,mol) and triethylamine (16.38 g, mol) were dissolved in dichloromethanein an ice bath and methacryloyl chloride (16.94 g, mol) was addeddropwise. The solution was allowed to stir overnight while the bathwarmed to room temperature.

Synthesis of an Acrylate Containing an Addition-FragmentationFunctionality (2-methylene-propane-1,3-di(thioethyl acrylate)(MDTA))_Under nitrogen,2-[2-(2-hydroxy-ethylsulfanylmethyl)-allylsulfanyl]-ethanol (12.60 g,mol) and triethylamine (15.92 g, mol) were dissolved in dichloromethanein an ice bath and acryloyl chloride (14.24 g, mol) was added dropwise.The solution was allowed to stir overnight while the bath warmed to roomtemperature.

Synthesis of 2-methylene-propane-1,3-di(thioethyl vinyl ether) (MDTVE)2-Methylene-propane-1,3-di(thioethylvinylether) (MDTVE) was synthesizedfrom 3-mercapto-2-(mercaptomethyl)-1-propene and 2-chloroethyl vinylether in a manner analogous to that described in the literature Evans,R. A. and Rizzardo, E. 2001, J. Polym. Sci A, Polym. Chem., 39(1),202-215). The crude oil was purified by bulb-to-bulb distillation.

Polymerizations involving divinyl ether monomers containing theaddition-fragmentation functionality can be partially decoupled fromstress relaxation by cationically photopolymerizing the divinyl ethercontaining the addition-fragmentation functionality while varying eitherthe concentration of a radical photoinitiator or using a radicalphotoinitiator absorbing at a different wavelength.

Conversion versus time was measured for a dimethacrylate containing anaddition-fragmentation chain transfer functionality (MDTMA) formulatedwith 0.25 wt % Irgacure 819 and irradiated at 400-500 nm, 5 mW·cm⁻² atroom temperature was found to be approximately 70% at 16 minutes. Thecorresponding conversion for tetraethylene glycol dimethacrylate(TETGDMA) was approximately 80% at 16 minutes. The irradiation wasstarted at t=1 minute.

EXAMPLE 5 Formulation of Tetrathiol/Divinyl Ether/MDTO Materials

The polymer studied was formed from 75 wt % of a stoichiometric mixtureof pentaerythritol tetra(3-mercaptopropionate) (PETMP) and2-methylene-propane-1,3-di(thioethyl vinyl ether) (MDTVE) and 25 wt % of2-methyl-7-methylene-1,5-dithiacyclooctane (MDTO) which produces arubbery crosslinked network with a T_(g) of −22° C. and a tensilemodulus at room temperature of 9.5 MPa. The monomers used to produce theTetrathiol/Divinyl Ether/MDTO materials are shown in Scheme 2. Monomersused were ((A) PETMP; (B) MDTVE; (C) MDTO) and (D)addition-fragmentation mechanism through the polymer backbone of theresultant polymer.

Resins were formulated with two photoinitiators, Irgacure 819 (aphenylphosphine oxide derivative active in the blue and near ultravioletfrom Ciba Specialty Chemicals) at 1.0 wt % and Irgacure 651 (a dialkoxyacetophenone derivative active in the near ultraviolet from Ciba) at 2.0wt %. The specimens were fully cured under irradiation for ten minutesat 400-500 nm at room temperature. The visible photoinitiator photolysesrapidly and photobleaches significantly during the curing stage, thusthe ultraviolet photoinitiator was included in the formulation to ensureresidual photoinitiator capable of introducing radicals in each specimenupon irradiation.

A light intensity gradient through the sample was produced byformulating the resin with an ultraviolet absorber (0.5 wt % Tinuvin 328a hydroxyphenylbenzotriazole derivative ultraviolet absorber from Ciba).A control network not containing the ultraviolet absorber was alsoproduced. Addition of a strongly UV absorbing compound to the standardformulation enabled the transmittance through the 0.15 mm thick fullycured samples to be readily decreased (from 57% transmittance at 365 nmfor a sample without Tinuvin 328 to 0.038% transmittance at 365 nm for asample with 0.5 wt % Tinuvin 328) without changing other materialproperties.

The rate of stress relaxation due to chain transfer through the allylsulfide functionalities increases at elevated functional groupconcentrations(Scott, T. et al., 2005, Science 308, 1615). A networkproduced from a stoichiometric PETMP/MDTVE mixture possesses one allylsulfide group per crosslink. The addition of 25 wt % MDTO to thePETMP/MDTVE mixture produces a network with approximately two allylsulfide groups per crosslink. Although an elastomer produced only from astoichiometric mixture of PETMP and MDTVE is able to produce the desiredstress relaxation, the addition of MDTO greatly enhanced the rate andextent of stress relaxation to emphasize the observed phenomenon.

EXAMPLE 6 Photoinduced Stress Relaxation in Tetrathiol/DivinylEther/MDTO Materials

The tetrathiol/divinyl ether/MDTO materials used were those described inExample 5. FIG. 5 shows the dependence of the rate of photoinducedstress relaxation for cured 75 wt % PETMP-MDTVE/25 wt % MDTO on theincident light intensity. In FIG. 5, the solid line denotes an intensityof 40 mW/cm², the dashed line 10 mW/cm² and the dotted line 4 mW/cm². InFIG. 5, a tensile strain of 0.060 was applied to these samples, followedby irradiation at 365 nm beginning at t=5 minutes.

The decrease in the overall stress relaxation rate in the samplecontaining 0.5 wt % Tinuvin 328 resulted from a reduction in the averagelight intensity. Though the average stress relaxation rate is lower, therelaxation rate at the incident side remains just as rapid. Uniquely,because the rate of stress relaxation is directly proportional to thelight intensity, the stress relaxation rate at the unirradiated side isreduced by 1500 times for a sample containing 0.5 wt % Tinuvin 328 whencompared with a sample without any UV absorber.

During the irradiation of the previously unirradiated side, the stressrelease results in the polymer bending away from the light anddecreasing the angle of curvature (θ in FIG. 1 a), as shown in FIG. 1 b.It is also possible to irradiate such a sample on the already-irradiatedside to increase the angle of curvature; however, extended exposure on asingle side depletes the initiator that is required to generate theradicals. Here, even at long exposure times, the sample never returnedto its original flat shape. The reasons for this are two-fold. Firstly,while stress is relieved preferentially on the irradiated side, thispreference is not perfect. As shown in FIG. 5, despite the highattenuation of light through the sample, more than half the initialstress was relieved over the course of the irradiation time. This meansthat stress was being relieved in more than just the exposed half of thesample. Thus, when the sample is irradiated on the previously unexposedside and stress is relieved predominantly on that side, stress is alsorelieved on the other side. Clearly, an optimal exposure time exists forinducing the maximum initial deformation and subsequent recovery wherethe stress has been largely relieved on the incident side while largelyremaining on the opposite side. Finally, there is a limit on the stressthat may be relieved as a small amount of residual stress remains, evenin transparent samples, following exposure. While the residual stress isslightly reduced in samples whose initial stress is lower, a smallamount of residual stress remains in each sample, likely due to thecomplete decomposition of the photoinitiator before the stress was fullyrelieved.

EXAMPLE 7 Stress Relaxation of Trithiocarbonate-Dimethacrylate-BasedDental Composites

Methods: A trithiocarbonate dimethacrylate (TTCDMA) monomer, capable ofundergoing radical-mediated RAFT, was mixed with 70 wt % BisGMA(bisphenylglycidyl dimethacrylate) and compared to a conventional dentalresin comprised of TEGDMA (triethylene glycol dimethacrylate) and 70 wt% BisGMA. The shrinkage stress and methacrylate conversion weresimultaneously measured during polymerization. The fracture toughnessand elastic modulus were measured to evaluate the effect of the TTCDMAmonomer on the mechanical properties. All the materials used herein wereevaluated as a composite, including 75 wt % silica fillers. ANOVA (CI95%) was conducted to assess the differences between the means.

Summary of Results: The TTCDMA composite exhibited a 65% stressreduction compared with TEGDMA-BisGMA though the reaction rate wasslower than the conventional dental composite, owing to the additionalRAFT reaction. The fracture toughness and elastic modulus of theTTCDMA-based composite were not significantly different than in theTEGDMA-based composite, while the T_(g) was decreased by 30° C. to155±2° C.

Significance: Despite only replacing the reactive-diluent, significantand dramatic stress reduction was observed while maintaining the elasticmodulus and fracture toughness. This new RAFT-capable monomer showsgreat promise to replace the reactive diluent in BisGMA-based dentalmaterials.

Introduction: Polymer-based composites have been vastly improved overthe past few decades; however, volume shrinkage and the associatedstress that evolve during curing remain a critical drawback, as theyleads to microcracking, microleakage, and secondary caries (Burke F J T,et al. Quintessence International, 1999; 30:234-242). During thepolymerization of dimethacrylate-based resins, which represent theprimary polymerizable component of dental composites, a large stressarises due to the nature of the monomer-polymer densification process.Once the resin undergoes a transition from a liquid-like to a solid-likematerial (i.e., sol-to-gel transition), the shrinkage stress is no longcapable of being dissipated and begins to build throughout the remainderof the polymerization. Methacrylate-based polymerizations areparticularly susceptible to stress development owing to both the lowgel-point conversion (Kleverlaan C J and Feilzer A J., Dent Mater, 2005;21:1150-1157) as well as the large amount of volume shrinkage per doublebond that occurs (Dewaele M, et al., Dent Mater, 2006; 22:359-365).

There have been several approaches to reduce stress (Bowman CNet al. JDent Res, 2011; 90:402-416) that utilize different polymerizablefunctional groups and/or different polymerization mechanisms to reducethe volume shrinkage and/or delay the gel point, including thiol-ene(Carioscia J A et al., Dent Mater, 2005; 21:1137-1143; Lu H, et al. DentMater, 2005; 21:1129-1136) and thiol-yne reactions (Fairbanks B D, etal. Macromolecules, 2009; 42:211-217; Chan J W, et al. Macromolecules,2010; 43:4937-4942), polymerization-induced phase separation (Lee T Y,et al., J Polym Sci Pol Chem, 2009; 47:2509-2517) and ring-openingpolymerizations. (Ge J H et al. Macromolecules, 2006; 39:8968-8976;Stansbury J W. et al. Polymers of Biological and BiomedicalSignificance, 1994; 540:171-183). The incorporation of a reversiblecovalent bond, which undergoes addition-fragmentation chain transfer(AFCT—e.g., see FIG. 6A), into a polymerizable material that leads tosignificant stress reduction has been reported (Kloxin C J et al.Macromolecules, 2009; 42:2551-2556; Scott T F, Draughon R B and Bowman CN, Advanced Materials, 2006; 18:2128−+; Scott T F et al., Science, 2005;308:1615-1617). The AFCT functional groups facilitate stress relaxationthroughout the polymerization by undergoing a non-degradative bondbreaking and reforming leading to rearrangement of network strands;(Kloxin et al., 2009; Scott et al. 2006; Scott et al, 2005) thus, thisapproach is focused on stress dissipation rather than reducing volumeshrinkage. The allyl sulfide AFCT functional group was incorporated inboth thiol-ene (Kloxin 2009) and thiol-yne reactions (Park H Y et al.Macromolecules, 2010; 43:10188-10190) to yield significantpolymerization stress reduction. Unfortunately, the effect is reduced innon-thiol containing resins, such as methacrylates, since the completereversibility of the allyl sulfide AFCT mechanism requires the presenceof thiyl radicals (see FIG. 6(A)). While allyl sulfide incorporationinto a methacrylate-based system does lead to reduced stress, the effectis significantly diminished with increasing methacrylate content (Park HY et al., Dent Mater, 2010; 26:1010-1016). The presence of thecarbon-centered radical in methacrylate homopolymerizations leads toirreversible AFCT of the allyl sulfide group (see FIG. 6(B)).

The trithiocarbonate functional group is frequently used as a reversibleaddition-fragmentation chain transfer (RAFT) agent to synthesizepolymers having low polydispersity (Mayadunne RTA et al.,Macromolecules, 2000; 33:243-245) Unlike the allyl sulfide functionalgroup, the trithiocarbonate functional group is capable of fullyreversible AFCT when reacting with a carbon-centered radical, such asthose present in a methacrylate polymerization (FIG. 6(C)). Here, theeffect of adding a trithiocarbonate dimethacrylate monomer to reducestress was evaluated by replacing the reactive diluent, TEGDMA, within aconventional BisGMA-TEGDMA (bisphenylglycidyl dimethacrylate/triethyleneglycol dimethacrylate) dental composite. In addition, fracture toughnessand elastic modulus were measured to compare the mechanical properties.All experiments were performed on the formulated composite, whichincludes 75 wt % silica fillers.

Materials: The monomers and photoinitiator used in this study are shownin FIG. 7. S,S′-bis[α,α′-dimethyl-α″-(acetyloxy)ethyl2-methyl-2-propenoate]-trithiocarbonate (TTCDMA, Trithio carbonatedimethacrylate) was synthesized from S,S′-bis(α,α′-dimethyl-α″-acetylchloride)-trithiocarbonate and 2-hydroxyethyl methacrylate (HEMA)following the procedure reported in the literature for reaction of thetrithiocarbonate with hydroxyl-terminated polydimethylsiloxane (PDMS)(Pavlovic D et al., J Polym Sci Pol Chem, 2008; 46:7033-7048). The crudeoil was purified by dissolving it in a 9:1 hexanes:ethyl acetate mixtureand subsequently filtering the insoluble impurities. Columnchromatography was performed using an 8:2 hexanes/ethyl acetatesolution. S,S′-bis(α,α′-dimethyl-α″-acetyl chloride)-trithiocarbonateproduct was made by the chlorination of theS,S′-bis(α,α′-dimethyl-α″-acetic acid)-trithiocarbonate with thionylchloride (Pavlovic 2008). S,S′-bis(α,α′-dimethyl-α″-aceticacid)-trithiocarbonate was prepared according to a previously publishedprocedure (Lai J T et al., Macromolecules, 2002; 35:6754-6756).Bisphenylglycidyl dimethacrylate (BisGMA, provided by Esstech,Essington, Pa.) and triethylene glycol dimethacrylate (TEGDMA, providedby Esstech) were used as received. Resins were composed of 70 wt. %BisGMA and 30 wt. % of either TEGDMA or TTCDMA. A phosphine oxide,phenyl bis(2,4,6-trimethyl benzoyl) (BAPO, BASF Corp., Florham Park,N.J.), was utilized at 1.5 wt % in the resins as a visible light-activephotoinitiator. 75 wt. % of silica filler (0.4 μm, Confi-Dental,Louisville, Colo.) was used to comprise the composite.

Methods Composite samples (2 mm thickness) were irradiated at 70 mW/cm²intensity with 400-500 nm light (Acticure 4000) for 20 minutes toobserve the conversion of the methacrylate functional group duringpolymerization. The methacrylate conversion was determined by monitoringthe infrared absorption peak centered at 6166 cm⁻¹ (C═C—H stretching,overtone) using Fourier transform infrared (FTIR) spectroscopy (Nicolet750). Specimens for fracture toughness test were prepared by photocuringcomposites at the same irradiation condition with the IR experiments ina cuboid mold (25 mm length*2.5 mm width*5.5 mm thickness) having a 2 mmrazor blade which is positioned vertically across the mold to form theinitial crack in the test specimen. After the specimens are separatedfrom the mold, the specimen surface was ground with sandpaper to createuniform dimension and remove defects. Fracture toughness was measured byusing a Mechanical Test System (MTS, The 858 Mini Bionix II Test System)using a 3-point bending test procedure with 20 mm span and 1 mm/minuterate. The elastic moduli (E′) and glass transition temperature (T_(g))of the polymerized samples were determined by dynamic mechanicalanalysis (DMA) (TA Instruments Q800). DMA experiments were performed ata strain and frequency of 0.1% and 1 Hz, respectively, and scanning thetemperature twice at ramp rate of 2° C./minute. The T_(g) was assignedas the temperature at the tan delta peak maximum (Ferrillo R G andAchorn P J, Journal of Thermal Analysis and calorimetry, 2000;60:377-390) of the second heating scan. This methodology does notmeasure the T_(g) of the as cured sample due to changes in conversionthat occur during the first thermal scan. Rather, the measurement isindicative of the maximum T_(g) achieved under these conditions. (ScottT F et al., Macromolecules, 2003; 36:6066-6074; Zhu S et al,Macromolecules, 1990; 23:1144-1150) Specimens used for DMA experimentswere prepared by sandwiching the uncured composite in a rectangular mold(2 mm gap) and irradiating under the same conditions used for the FTIRexperiments. The shrinkage stress was monitored using tensometry(Paffenbarger Research Center, American Dental Association HealthFoundation) (Lu, 2005; Lu H et al, Journal of MaterialsScience-Materials in Medicine, 2004; 15:1097-1103), which was equippedwith optical fibers which enable simultaneous monitoring of the reactionprogression via FTIR spectroscopy. Uncured composite was injectedbetween two glass rods which are positioned in a 9 cm beam length of thestainless steel beam. Samples were covered with a plastic sheath toprevent oxygen inhibition of the methacrylate during polymerization.ANOVA (CI 95%) was conducted to determine differences between the meansfor all the reported results.

Results: Fracture toughness and DMA measurements of the BisGMA-TEGDMAand BisGMA-TTCDMA composites are presented in FIG. 8. The elasticmodulus of the TTCDMA-based composite is slightly lower than for theTEGDMA-based composite over the temperature range from 10° C. to 100° C.As the glass transition of the TTCDMA-based composite is lower than theTEGDMA-based composite, the TTCDMA-based composite also exhibits anelastic modulus decreases at correspondingly lower temperatures. Theaverage fracture toughness value of the as-cured TTCDMA sample washigher than but not statistically different from the TEGDMA control atthe 95% confidence level (FIG. 8A).

The reaction kinetics for the BisGMA-TEGDMA and BisGMA-TTCDMA compositeswere monitored for 20 minutes during irradiation (FIG. 9). Themethacrylate conversion in the TTCDMA-based composite was lowerinitially; however, the final conversion was the same as theTEGDMA-based control system. The shrinkage stress evolution of theTTCDMA-based composite is slower than that of the TEGDMA-basedcomposite. While both composites exhibited similar final conversionvalues, the final shrinkage stress of the TTCDMA-based system was 65%lower than the stress level of the TEGDMA-based composite (FIG. 10A).Table 2 presents a summary of the methacrylate conversion, stress,T_(g), elastic modulus (E′), and fracture toughness for BisGMA-TEGDMA70/30 wt % and BisGMA-TTCDMA 70/30 wt % composites. The composite isformulated with 75 wt % filler and 25 wt % resin. Resins includes 1.5 wt% of BAPO as a visible light initiator and are exposed to 400-500 nmlight at 70 mW/cm² for 20 minutes. In each of the experiments, valuesfollowed by the same letter in the same column are not significantlydifferent using an ANOVA test with a 95% confidence level.

Discussion: The TTCDMA monomer was designed to have a similar molecularstructure with TEGDMA, targeting its use as a reactive diluent toreplace TEGDMA while simultaneously adding RAFT capability to promotestress relaxation. Since the RAFT reaction is favorable when the leavingradical group is stable, the TTCDMA core, which replaces ethyleneglycol, was designed to have a dimethyl substituted carbon adjacent tothe trithiocarbonate; therefore, the carbon-sulfur fragmentation product(FIG. 6C) generates a more stable tertiary carbon radical. (Lai, 2002)However, the molecular weight of TTCDMA is significantly larger thanTEGDMA and thus the crosslink density of the network is slightly reducedat an equivalent conversion. Nevertheless, while the incorporation ofthe TTCDMA decreases the T_(g) compared with the control composite, itsutilization does not affect the fracture toughness of the material (FIG.8A). More importantly, the TTCDMA-based composite, despite containingonly 30% of monomers with the RAFT core, exhibits a much lower stress ascompared with the TEGDMA-based composite (FIG. 10A). Reducedpolymerization rate can lower the final conversion; however, here, theTTCDMA-based composite exhibited an equivalent methacrylate conversionrelative to the TEGDMA-based composite.

As shown in FIG. 10B, the origin of the stress is also significantlydifferent in these two systems due to the AFCT mechanism. Inconventional dimethacrylate polymerizations, the stress begins to riseat very low conversions; however, the stress in the TTCDMA-basedcomposite begins to rise only after nearly 40% conversion ofmethacrylate groups. This behavior is in stark contrast to theTEGDMA-based composite where the stress begins to increase before 10%methacrylate conversion. It is clear that the incorporation of aRAFT-capable functional group, particularly one such as thetrithiocarbonate that is capable of multiple reversible reactions with amethacrylate radical, into a dimethacrylate polymerization, even in lowamounts, dramatically alters the stress evolution behavior in thesesystems. This outcome represents a new paradigm to consider inmethacrylate-based dental restorative materials for preserving overallnetwork mechanics while reducing stress levels.

Conclusion: BisGMA-TEGDMA and trithiocarbonate-containingBisGMA-TTCDMA-based composites were investigated to demonstrate stressrelaxation via RAFT. The trithiocarbonate functional group wasimplemented to successfully induce RAFT which led to networkrearrangement. Ultimately, the inclusion of this mechanism resulted in a65% stress reduction as compared to the standard BisGMA-TEGDMAcomposite. Fracture toughness of the TTCDMA-based composite was slightlyhigher though not significantly different from the TEGDMA-basedcomposite even though the TTCDMA-based composite has dramaticallyreduced stress. In summary, the RAFT mechanism for stress relaxation viathe inclusion of trithiocarbonates and similar RAFT moieties haspotential for replacing conventional dimethacrylate materials withnearly identical mechanical properties but stress levels that are afraction of current composites.

TABLE 2 Fracture Methacrylate Stress E′ at 25° C. Toughness Systemsconversion [MPa] T_(g) [° C.] [Gpa] [MPa · m^(0.5)] BisGMA-TEGDMA 0.67 ±0.003^(a) 1.7 ± 0.04^(a) 184 ± 1^(a) 15.3 ± 2^(a) 1.18 ± 0.09^(a)BisGMA-TTCDMA 0.68 ± 0.02^(a) 0.6 ± 0.02^(b) 155 ± 2^(b) 13.7 ± 0.7^(a)1.24 ± 0.09^(a)

1) A mixture for forming a dental material comprising a crosslinkednetwork of polymer chains, the mixture comprising a) a first monomercomprising a reversible addition-fragmentation chain transferfunctionality in the monomer backbone and a plurality of acrylate ormethacrylate groups; b) a second monomer comprising a plurality ofacrylate or methacrylate groups and not comprising a reversibleaddition-fragmentation chain transfer functionality; and c) a source offree radicals wherein the addition-chain transfer functionality is athiocarbonylthio group. 2) The mixture of claim 1, wherein the molarratio of addition-fragmentation chain transfer functionalities toacrylate and methacrylate groups is from 1 % to 15%. 3) The mixture ofclaim 1, wherein the molecular weight of the first monomer is from400-800 amu and the molecular weight of the second monomer is from200-800 amu. 4) The mixture of claim 1, wherein theaddition-fragmentation chain transfer functionality is atrithiocarbonate. 5) The mixture of claim 4, wherein the first monomeris described by

wherein R₄, R₅, R₆, and R₇, independently, are linear or branched alkylhaving from 1 to 6 carbon atoms or aryl, L₁ and L₂, independently, arealkylene or ether, and PG₁ and PG₂ are acrylate or methacrylate. 6) Themixture of claim 5, wherein PG₁ and PG₂ are methacrylate groups. 7) Themixture of claim 6, wherein L₁ is —(CH₂—CH₂₋O)_(g)— and L₂ is—(O—CH₂—CH₂)_(g)— where g is an integer from 1 to
 6. 8) A dentalrestorative composition comprising a) a multifunctional acrylate ormethacrylate monomer comprising a reversible addition fragmentationchain transfer functionality in the monomer backbone, b) amultifunctional methacrylate monomer or oligomer comprising a bisphenolA derived group or a urethane group and not comprising a reversibleaddition fragmentation chain cleavage group, c) filler particles havingan average size from 0.005 to 5.0 micrometers. d) a free radicalinitiator wherein the addition-chain transfer functionality is athiocarbonylthio group, the amount of filler particles is from 45 to 85%by weight of the composition and the molar ratio ofaddition-fragmentation chain transfer functionalities to acrylate andmethacrylate groups is from 1% to 15%. 9) The composition of claim 8,wherein the addition-fragmentation chain transfer functionality is atrithiocarbonate. 10) The composition of claim 9, wherein the monomer ofstep a) is described by

wherein R₄, R₅, R₆, and R₇, independently, are linear or branched alkylhaving from 1 to 6 carbon atoms or aryl, L₁, L₂, independently, arealkylene or ether and PG₁ and PG₂ are acrylate or methacrylate. 11) Thecomposition of claim 10, wherein PG₁ and PG₂ are methacrylate groups.12) The composition of claim 11, wherein L₁ is —(CH₂CH₂O)_(g)— and L₂ is—(OCH₂CH_(2)g)— where g is an integer from 1 to
 6. 13) The dentalrestorative composition of claim 8, wherein the multifunctionalmethacylate monomer of step b) is a bisphenol A-based dimethacrylate ora urethane dimethacrylate. 14) A method of making a polymeric compositematerial, the method comprising the steps of: a) forming a mixturecomprising i) a first monomer comprising a reversibleaddition-fragmentation chain transfer functionality in the monomerbackbone and a plurality of acrylate or methacrylate groups, wherein theaddition-chain transfer functionality is a thiocarbonylthio group; ii) asecond monomer comprising a plurality of acrylate or methacrylate groupsand not comprising an reversible addition-fragmentation chain transferfunctionality; iii) a source of free radicals; and iv) filler particleshaving an average size from 0.005 to 5.0 micrometers; and b) subjectingthe mixture to polymerization conditions for a time sufficient toachieve acrylate and methacrylate group conversion of at least 50%wherein the molar ratio of addition-fragmentation chain transferfunctionalities to acrylate and methacrylate groups in the compositionis from 1% to 15%. 15) The method of claim 14 wherein the molecularweight of the first monomer is from 400-800 amu and the molecular weightof the second monomer is from 200-800 amu. 16) The method of claim 15,wherein the second monomer comprises at least one bisphenol A-derivedgroup 17) The method of claim 15, wherein the second monomer comprisesat least one urethane group. 18) The method of claim 15, wherein theaddition-fragmentation chain transfer functionality is atrithiocarbonate. 19) The method of claim 18, wherein the first monomeris described by

wherein R₄, R₅, R₆, and R₇, independently, are linear or branched alkylhaving from 1 to 6 carbon atoms or aryl, L₁, L₂, independently, arealkylene, or ether and PG₁ and PG₂ are acrylate or methacrylate. 20) Themethod of claim 19, wherein PG₁ and PG₂ are methacrylate groups. 21) Themethod of claim 19, wherein L₁ is —(CH₂CH₂O)_(g)— and L₂ is—(OCH₂CH₂)_(g)— where g is an integer from 1 to 6.